1. Field of the Invention
The present invention relates to a focal plane shutter used in a camera.
2. Description of Related Art
In recent years, due to demands for improvement of film sensitivities and new image expressions, it has been necessary to move toward higher speeds in terms of camera shutter speeds and strobe alignment speeds, and cameras have been realized with shutter speeds as fast as 1/80000 second and alignment speeds as fast as 1/250 second. With this type of focal plane shutter, fast shutter speeds that exceed the strobe alignment speed are produced by moving the front blind (or curtain) and rear blind (or curtain) with shifted timing and by carrying out slit exposure with a gap (slit) between the first blade of the front blind and the first blade of the rear blind being fixed at a certain gap distance, or with the gap being varied according to the speed.
The shutter includes four or five dividing blades (light shielding blades) and a driving mechanism that drives the blades. The material from which these light shielding blades are formed can be fiber reinforced plastic (referred to hereafter as FRP), aluminum, or the like. Alternatively, combinations of the above can be used, such as, when four blades are used, the two blades with the greatest amount of movement can be formed from FRP, while the two blades with the least amount of movement can be formed from aluminum.
FRP, as disclosed in Japanese Laid-Open Patent Publication Sho 59-61827, uses carbon fibers aligned in one direction as the reinforcing fibers, and uses epoxy resin as a matrix resin. FRP has multiple layers of prepreg sheets (which are intermediate products) with the direction of the fibers of each sheet being perpendicular or virtually perpendicular to that of the sheets on either side. FRP is manufactured by pressing the prepeg layers together and applying heat, thus causing them to be vulcanized. FRP material that has been thus manufactured is cut into the desired light shielding blade shape. The cutting is generally done by stamping with a press.
Blades made from FRP are lightweight and have a high bending rigidity. At high shutter speeds of 1/8000 second, shaking or oscillation of the blades during movement and directly after stopping is extremely small, and even if the next shutter action is carried out while the blade is shaking, the blade will not collide with the other blades or with the aperture (the device that determines the field angle) and be damaged, the shutter will not become inoperable, and hence it becomes possible to achieve an extremely high durability. Even if there is temporary shaking with FRP, the shaking condition is quickly absorbed, when compared, for example, to an aluminum-type blade.
However, the diameter of the carbon fibers is 3 to 8 .mu.m, and if the alignment is bad, there will be localized increases or decreases in the number of fibers, causing non-uniformity in strength and irregularities in flatness. In addition, blades warp for various reasons when used for extended periods of time. Because these types of blades have large variations in thickness, they are discarded during manufacturing before assembly into the camera. Blades that appear inferior are also discarded, such as those that have experienced failures in layering or that have scratches or the like. Furthermore, a number of blades warp when a coating is applied, which blades are also discarded. Thus, FRP-type blades are extremely time consuming from the standpoint of quality control, and the acceptance rate is low. Moreover, FRP-type blades are extremely expensive.
In the case of aluminum-type blades, because rejects and problems in quality control are extremely small when compared with FRP, aluminum blades have a cost advantage over FRP. However, the weight of the blades becomes fairly heavy, which increases the charge energy. Additionally, shaking during movement and directly after stopping is extremely large, and because this shaking condition is not easily absorbed, the durability is markedly inferior to a shutter with FRP-type blades.
Recently, it has come to be realized that, under specialized usage conditions in which photography is performed while the camera is moving, namely, in conditions in which the shutter is released while a strong acceleration is applied in a direction nearly perpendicular to the direction of travel of the blades, there is a strong possibility that the blades will be damaged by colliding with an intermediate plate and/or a light shielding plate that separates the front blind and rear blind, even if the blades are formed from FRP.
More recently, even faster super-high speed shutters are being requested that have speeds exceeding 1/8000 second. In order to respond to these demands, it is necessary not only to prevent the collision and damage problems described above, but also to minimize increases in the charge energy, while further improving the quality of these products and keeping costs down.